This board locks up if we stress test the eMMC, as the regulator s4 is
unable to supply enough current for all the peripherials attached to it.
As this supply is wired up to most of the peripherials including DDR,
it resulted in such lockup.

This patch fixes this issue by setting s4 regulator correctly with
Auto power mode.
Reported-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
[Srinivas Kandagatla: rewrote the change log]
Tested-by: Girish Sharma <girish.sharma@einfochips.com>
Signed-off-by: Parth Pancholi <parth.pancholi@einfochips.com>
Signed-off-by: Andy Gross <andy.gross@linaro.org>